FIR CFG: handle 'Dead' properly in edge kind merging

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/dfa/cfg/ControlFlowGraph.kt

@@ -72,13 +72,17 @@ sealed class CFGNode<out E : FirElement>(val owner: ControlFlowGraph, val level:
         }
 
         private fun merge(first: EdgeKind, second: EdgeKind?): EdgeKind? {
-            return when (first) {
+            return when (second) {
-                second -> first
+                null, EdgeKind.Simple, first -> first
+                EdgeKind.Dead -> second
+                // Note: first can be only Cfg, Dfg, or Dead
+                else -> when (first) {
                     EdgeKind.Cfg -> if (second == EdgeKind.Dfg) null else first
                     EdgeKind.Dfg -> if (second == EdgeKind.Cfg) null else first
                     else -> first
                 }
             }
+        }
 
         internal fun addJustKindEdge(from: CFGNode<*>, to: CFGNode<*>, kind: EdgeKind, propagateDeadness: Boolean) {
             if (kind != EdgeKind.Simple) {